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AGNT40 KWNM 250727

.Marine Weather Discussion for N Atlantic Ocean
.NWS Ocean Prediction Center Washington DC
215 AM EDT Sat 25 Mar 2017

.Forecast Discussion...Major features/winds/seas/significant
.weather for the North Atlantic Ocean W of 50W from 30N to 50N.

The most recent Ascat overpasses across the offshore waters from
0050Z to 0230Z indicated southwest winds to 30 kt across the
northern NT2 zones and to 25 kt elsewhere north of Hatteras
Canyon. Although the most recent model guidance all initialized
some winds to 30 kt, Ascat shows more widespread 30 kt than
initialized. However, the higher 00Z GFS first sigma level winds
are too high. As a result, and after some testing some different
model blends, came up with a 2:1 00Z GFS first sigma level and
00Z ECMWF boundary layer wind blend that best matched the Ascat.
At 06Z the cold front had moved off the New England coast and
the 00Z models are in good agreement that the front will slowly
move south across the NT1 and northern NT2 waters today into
tonight, before stalling off the Delmarva peninsula late tonight
into early Sunday. Models are also consistent that the west to
southwest widns south of the cold front will gradually diminish
through the day today, to less than 25 kt later this evening.
Then as high pressure builds off the New England coast tonight
into Sunday night, the 00Z GFS/NAM/Canadian are somewhat
stronger than the 00Z ECMWF/UKMET with the easterlies just
poleward of the stationary front across the southern NT1 and
northern NT2 waters Sunday into Sunday night. This discrepancy
in the models` wind speeds north of the front persists late
Sunday night into Monday night as the front lifts northeast back
across the NT1 waters as a warm front. OPC preference continues
with these slightly higher winds to 25 kt. For wind grids will
use the blend noted above through tonight, before transitioning
to the 00Z GFS Sunday through Monday.

In the medium range, the 00Z models diverge greatly with a
series of weak upper shortwaves moving through the northeastern
US and off the northern mid Atlantic and/or New England coasts
Tuesday night and Wednesday. The GFS has been consistently
stronger with the associated developing low and cold front
forecast to move offshore late Tuesday and Wednesday. There also
continued to be farily widespread differences with the timing of
this system. The 00Z models then become more consistent later
Wednesday night into Thursday that the aformentioned shortwaves
and approaching northern stream trough will consolidate into an
upper low across southeastern Canada and Maritimes. Given model
discrepancy, have decided to use a blend of the 00Z GFS and 00Z
ECMWF for wind grids Tuesday night through Wednesday night,
which offers a compromised timing with the front. However did
adjust these blended winds upward by about 10 percent to get
some associated winds to 25 kt.  We have above average forecast
confidence that winds across the NT1 or NT2 offshore waters
should remain below gale through at least midweek.

.Seas...The 00Z Wavewatch III is slightly better initialized
than the 00Z ECMWF WAM with wave heights across the west
Atlantic this morning. However, the 00Z WW3 may not be high
enough across the nothern NT2/southern NT1 zones where there are
still some 30 kt winds occurring. Am preferring to use an even
blend between this guidance through the forecast period.

.Extratropical Storm Surge Guidance...N/A.


.NT1 New England Waters...

.NT2 Mid-Atlantic Waters...


.Forecaster Clark. Ocean Prediction Center. is the U.S. government's official web portal to all federal, state and local government web resources and services.